In general, a nitride semiconductor is utilized or studied for a light-emitting device or a high-power semiconductor device. In the case of the nitride semiconductor light-emitting device, a quantum well layer emitting light in an emission layer is made of InGaN containing indium, and light-emitting devices for a wide color range from blue to orange can be prepared by varying the In content. In recent years, a blue or green light-emitting diode or a bluish purple semiconductor laser has been developed utilizing the characteristics of the nitride semiconductor light-emitting device. Japanese Patent Laying-Open 10-270804 reports a light-emitting device including an emission layer containing a GaNAs well layer/a GaN barrier layer.
In the case of preparing a light-emitting device utilizing an InGaN quantum well layer, however, the chemical thermal equilibrium state of the InGaN layer is so unstable that it is difficult to form an emission layer having excellent crystallinity. Particularly in the case of growing an InGaN crystal layer containing In of at least 15% in the group III elements, the InGaN crystal is readily phase-separated into regions having high and low In contents respectively depending on its growth temperature (this phenomenon is hereinafter referred to also as concentration separation). Such concentration separation causes reduction of luminous efficiency and increase of the half-width of the emission wavelength (color heterogeneity).
On the other hand, a GaNAs well layer (As is at least partially replaceable with P and/or Sb) containing no In does not cause the aforementioned problem of concentration separation, but causes phase separation containing different crystal systems (separation of a hexagonal system and a cubic system) due to As etc. contained therein, and such crystal system separation causes reduction of crystallinity and luminous efficiency of the well layer.
Accordingly, a principal object of the present invention is to improve luminous efficiency of a nitride semiconductor light-emitting device including an emission layer containing a quantum well of nitride semiconductor by improving crystallinity and suppressing phase separation of the quantum well layer.